Amorphous (non-crystalline) oxide semiconductors used in a TFT have a high carrier mobility and a large optical bandgap as compared to generalized amorphous silicon (a-Si), and can be deposited at low temperature. Thus, the amorphous oxide semiconductors are expected to be applied to next-generation display devices required for large size, high resolution, and high-speed driving, as well as resin substrates with a low heat resistance, and the like. In formation of the above oxide semiconductor (film), a sputtering method is preferably used which involves a sputtering target made of the same material as the film. The thin film formed by the sputtering method has excellent in-plane uniformity of the composition or thickness in the direction of the film surface (in the in-plane direction) as compared to thin films formed by ion plating, vacuum evaporation coating, and electron beam evaporation. The sputtering method has an advantage that can form the thin film of the same composition as that of the sputtering target. The sputtering target is normally formed by mixing, sintering, and mechanically processing oxide powders.
The compositions of the oxide semiconductor used in the display device include, for example, In-containing amorphous oxide semiconductors, such as “In—Ga—Zn—O, In—Zn—O, and In—Sn—O (ITO)” (see, for example, Patent Literature 1).
A ZTO-based oxide semiconductor formed by adding Sn to Zn to be made amorphous has been proposed as an oxide semiconductor which can reduce material costs because of the absence of expensive In and which is appropriate for mass production. However, the ZTO-based oxide semiconductor manufactured by a conventional pressureless sintering method tends to have a non-uniform high specific resistance, which often causes abnormal discharge during sputtering. For this reason, for example, Patent literature 2 has proposed a method for suppressing the occurrence of abnormal discharge or cracking during sputtering by controlling a composition of an oxide semiconductor not to contain a tin oxide phase by burning for a long time. Patent literature 3 has proposed a method for suppressing the abnormal discharge during sputtering by performing two-stage processes, namely, a temporary powder burning process at a low temperature of 900 to 1300° C. and a main powder burning process to increase the density of a ZTO-based sintered body.